Adding functionality to X10 networks with 802.15.4. Using 802.15.4 to communicate and add functionality to X10 wired networks. William Wallace, Jose I. Moreno, Ruben Hidalgo Universidad Carlos III de Madrid, Dpto. Ing. Telemática Avda. Universidad 30, 28911 Leganés, Madrid, Spain {williamdaniel.wallace, joseignacio.moreno, ruben.hidalgo}@uc3m.es Abstract. Although several newer technologies exist in the domotics market, one of the oldest remains extremely popular. Despite of its very limited functionality and performance, X10 is still a very common technology to manage simple Home Networks. Even if other wired solutions such as KNX/EIB or LonWorks out perform X10 in every single aspect, they might still be a bit of an overkill when addressing simple needs in a modest Home Network. However, X10 lacks support for wireless networks that could add a lot of functionality to its wired network. A good example would be how sensor networks could communicate with already existing X10 modules to allow full home automation in, for example, light and shutter control. This paper will focus on the use of 802.15.4 to add functionality to simple X10 networks, in particular how 802.15.4 could allow the use of remote controls (or switches) acting over regular X10 modules, and how X10 switches can act over wireless modules using 802.15.4 as the transport layer. We will also see how 802.15.4 doesn't add complexity in the deployment and management of X10 Home Networks, which is one of the reasons such an old technology remains popular. Keywords: 802.15.4, X10, Java, ZigBee, Domotics. 1 Introduction X10 [1] is one of the oldest, yet still fairly popular, domotics technologies. Developed in 1975, it remains to date one of the references in home networking by using the Power Line as the Physical mean to communicate different modules. X10 has a very modest performance, close to just one command per second, and isn't reliable, this means there's no way to know if a command has reached destination or not. But still, X10 is a pretty cost effective solution and its functionality is enough for the modest needs of a small home: control over lights, shutters, alarms and even climate conditions. However, X10 lacks a good wireless companion. There's no real standard for wireless communication using X10, and most solutions don't really offer a true wireless network solution, but more the ability to act over wireless modules separately. Companies such as Insteon [2] do offer wireless compatibility, but still lack the strength of a real standard behind them. IEEE 802.15.4 [3] provides a cost effective, standard solution for this. 802.15.4 is a low cost, with extremely low power consumption and low data rate technology released in 2004, and has the strength of a standard defined by the IEEE organization. It's designed to work very well in sensor networks, which is an asset X10 lacks, because even if it is compatible with sensor networks, its limitations don't really allow it to manage complex sensor networks that could really improve the domotics service it provides [4]. Other technologies offer much better performance, or even lower cost, if deployed from scratch, but still many X10 networks already exist that could be greatly improved by using a simple and cost effective wireless solution. 2 X10 Overview X10 is one of the oldest domotics technologies. Through the Power Line network, it allows the communication of up to 256 modules, divided in 16 Areas with 16 devices per Area. Therefore, a module's address is composed of an area code (ranging from A to P) and a device code (ranging from 1 to 16). X10 isn't reliable, this means there's no way to know if a command has been correctly received or not. In order to avoid undesired effects (for example, a shutter going down when it shouldn't), each command is sent twice, and in case the copies don't match nothing is done. A command is divided in two parts before being sent. The first part addresses one to several modules and tells them to listen for the command to be sent afterwards. Then, the function these devices have to execute is sent. Since 3 cycles have to pass before the second part is sent, each is sent twice to avoid errors and 3 cycles have to go by before another command is sent, each command takes about 50 cycles. Fig. 1. X10 telegram structure X10 has a very low performance by today's standards, since it sends "bits" of information on the zero crossing of the power line's AC signal. Since the AC signal's frequency in Europe is 50 Hz, and each simple command needs about 50 cycles, the performance is at best one command per second. On the other hand, X10 is very inexpensive and simple, which is the rea-son many users still choose it as a solution for their home automation needs. In fact, X10 only supports up to 16 commands, of which 3 of them are extended commands (one for security, another one for sensor networks and advanced func-tionality and the last one is reserved for the manufacturer). However, only 13 commands are normally used: • ON: Turns a device On. • OFF: Turns a device Off. • DIM: Dims a light or lowers the shutter by 5%. • BRIGHT: Brightens a light or raises a shutter by 5%. • ALL LIGHTS OFF: Turns off all light modules of an area code. • ALL LIGHT ON: Turns on all light modules of an area code. • ALL UNITS OFF: Turns off all units in an area code. • STATUS ON. • STATUS OFF. • STATUS REQUEST. • HAIL REQUEST. • HAIL ACKNOWLEDGMENT. 3 IEEE 802.15.4 Overview Released in 2004, the IEEE 802.15.4 standard defines a low cost, low data rate (up to 256Kbps) and very low power consumption communications technology. It's aimed at small devices, powered by batteries which should last up to several months or even years. Because of this, data communication must be brought to a minimum if batteries must last, so even if the data rate may be enough for the transmission of multimedia content, it's strongly recommended not to use 802.15.4 networks for this purpose, since the relatively high data rate is useful to bring the time the 802.15.4 antenna is powered to a minimum, thus greatly reducing power consumption. 802.15.4 allows up to 16 channels on the 2.4 GHz band, which is useful to avoid interference with widespread WiFi and Bluetooth / WiBree networks [5], which also use the same band to communicate. Although 802.15.4 is specially useful in sensor networks, its application in other scenarios is also interesting. Take for example a 802.15.4 light switch, light switches are not activated extremely often, maybe just a dozen times a day or so. By using 802.15.4, a light switch may be placed wherever we want, at a very small cost (around 3 euros per 802.15.4 module, plus the cost of the switch itself). Compare this to the cost of placing a regular, power line connected, light switch. Its fairly obvious that by the use of this standard, not only new functionality may be achieved in domotics networks, but could also lower the cost of common home networking needs. 802.15.4 allows for two types of devices, Full Function Devices (FFDs), used as network coordinators and Bridges to other networks, and Reduced Function Devices (RFDs), that can only communicate with their assigned FFD. In a domotics environment, it is clear a star topology might be the best idea. 4 System Concept Device C 8 Light Bulb 1 Light Bulb 2 0 2 .1 5 .4 Gateway X10 Network Device B Appliance Appliance Switch 1 Switch 2 1 2 Device A PC Fig. 2. Overview of the Network Structure The network shown in figure 2 will allow us to illustrate how 802.15.4 devices may allow modules connected to them to communicate with X10 net-works as if they were directly plugged to them. In this scenario, Device B is equipped with four switches and four LED, and we will show how these switches can control X10 modules, and how the X10 Switches may act on the LED. Device C is the PAN coordinator, and acts as a bridge to the X10 wired network. Device A provides an 802.15.4 connection to the PC. In order to implement this network, a simple X10 protocol has been developed to communicate wireless devices with the X10 wired network. This protocol allows modules connected to a 802.15.4 device to be treated as if they where directly connected to the Power Line, as well as allowing the PC to communicate with the Gateway as if it were directly connected to it, allowing network monitoring as well as device management. 5 A Simple X10 Protocol in 802.15.4 Fig. 3. Protocol Architecture in 802.15.4 X10 networks The X10 protocol will be implemented directly over 802.15.4's MAC layer. This layer provides device discovery functionality, so all the upper X10 layer has to provide is the necessary logic in order to identify different modules connected to the same device. Since the protocol architecture is very simple, so will be the necessary software to be developed on 802.15.4 devices, and the data-grams to be sent are kept as small as possible in order to profit from 802.15.4 low power consumption.